Passive Frequency References

In passive frequency standards, the atomic resonance is probed with an external local oscillator (LO), such as a quartz crystal oscillator combined with a frequency synthesizer. The frequency of the local oscillator is compared to the atomic resonance frequency and corrected with a feedback loop if a difference exists. The the two frequencies are compared in three steps. First the atoms are prepared in a specific quantum state, usually by optical pumping with a laser or lamp. For vapor-cell microwave frequency references, this would be the magnetically-insensitive Zeeman level of the lower-energy hyperfine-split ground state. Once the atoms are prepared, the local oscillator frequency is applied to the atoms (either using direct microwave excitation or with CPT), causing transitions to occur between the state in which the atoms are prepared and a second state, differing in frequency (energy) by roughly the LO frequency. When the LO frequency is different from the atomic resonance frequency, few atoms are excited from the initial level to the final level. but when the LO frequency exactly equals the atomic resonance frequency, many atoms are excited. This excitation can be detected by monitoring the absorption of the laser or lamp used to prepare the atoms in the first state. The three steps are shown pictorially below.

The operation of a passive atomic frequency reference. Atoms are prepared in a certain quantum state and resonantly excited with a signal generated by a local oscillator (LO). After excitation, the new state of the atom is detected. When the local oscillator is not resonant with the atoms, the state has a small probability of changing. When the LO is resonant, the probability is large.

The entire clock may therefore be thought of as having three components: a physics package, which contains the atoms, a local oscillator, which generates the initial frequency, and a control circuit, which stabilizes the LO frequency to the atomic resonance. Typically, the LO is quite stable over short times but drifts significantly over long periods (more than several seconds) due to environmental and mechanical effects. The role of the atoms, therefore is to stabilize the LO frequency over long periods. This is shown graphically in the plot below at right.

In a passive frequency standard, a local oscillator probes the atomic transition and is locked to the atomic resonance frequency with a control circuit. When locked, the output frequency of the LO becomes very stable.

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References:

J. Kitching, "Local oscillator requirements for chip-scale atomic clocks," unpublished.

J. Kitching, S. Knappe, N. Vukicevic, L. Hollberg, R. Wynands, and W. Weidemann, "A microwave frequency reference based on VCSEL-driven dark line resonance in Cs vapor," IEEE Trans. Instrum. Meas., 49, 1313-1317, 2000.